1. Field of the Invention
The present invention relates to an electrolytic solution for an electrolytic capacitor that enables to give an aluminum electrolytic capacitor having low impedance properties and a long service life. The present invention also relates to an aluminum electrolytic capacitor with low impedance properties and a long service life utilizing the electrolytic solution and/or an electrode that is proof against deterioration.
2. Description of the Related Art
An aluminum electrolytic capacitor has a constitution that houses in a sealing package an anode constructed from aluminum foil with an aluminum oxide film on the surface, a cathode constructed from aluminum foil, and a separator holding an electrolytic solution located between the anode and the cathode. The capacitors with a wound or laminated structure are widely used.
What are conventionally known as the electrolytic solutions for activating a small, low voltage aluminum electrolytic capacitor are one with ethylene glycol as a main solvent and a carboxylic acid such as adipic acid and benzoic acid or a carboxylic acid such as an ammonium salt as an electrolyte, and one with γ-butyrolactone as a main solvent and a quaternary cyclic amidinium salt or the like of a carboxylic acid such as phthalic acid and maleic acid as an electrolyte.
With downsizing of electronic devices in recent years, low impedance properties are demanded for such aluminum electrolytic capacitor. The low impedance properties are also required in using the capacitor under high frequency conditions. To meet these requirements, use of a highly conductive electrolytic solution with a low specific resistance is desirable, and studies based on the electrolytic solution with ethylene glycol as s main solvent and a carboxylic acid and/or a carboxylic salt as electrolytes have been conducted to lower a specific resistance by increasing the water content in the electrolytic solution.
However, water as well as the carboxylic acid and/or the carboxylic salt as the electrolytes in the electrolytic solution is chemically active substance for the anode and the cathode that are constructed from aluminum foil. The aluminum oxide film on the surface of the electrode dissolves by reaction with carboxylic acid anions, and carboxylic acid complexes of aluminum are formed. Additionally, when water reaches an aluminum part through the aluminum oxide film on the surface of the electrode, aluminum dissolves, hydroxides of aluminum are formed, and concurrently with this reaction, hydrogen gas is generated. Therefore, there is a problem that, when the water content in the electrolytic solution is increased, the electrode foil deteriorates, a leakage current of the capacitor increases, and a service life of the capacitor is reduced. Especially, in a high-temperature life test at 105° C. or more, when the water content exceeds 15% by weight of the solvent, hydrogen gas is massively generated by the above-mentioned reaction in an abrupt manner, and a pressure within the capacitor increases by the gas. This results in opening of a safety valve, and the capacitor becomes unusable.
Addition of phosphoric acid to the electrolytic solution in order to prevent this deterioration of the electrode foil is conventionally known. If a moderate amount of phosphate ions exists in the electrolytic solution, the dissolution of aluminum in the anode and the cathode as well as the formation of the hydroxides and other reaction products is inhibited, and the generation of hydrogen gas is also inhibited.
However, when phosphoric acid is added to the electrolytic solution, phosphate ions are combined with aluminum ions that are eluted into the electrolytic solution and form compounds that do not dissolve in the electrolytic solution. These insoluble compounds then attach to the electrode foil and the phosphate ions disappear from the electrolytic solution. Therefore, preventing the deterioration of the electrode foil by the phosphate ions is not sufficient. Additionally, if an added amount of phosphate acid in the electrolytic solution is excessive, the aluminum oxide film on the surface of the electrode dissolves by reaction with the phosphate ions and the leakage current of the capacitor increases. To deal with these problems, the applicants have taken into consideration the fact that capacitor properties are well maintained while the adequate amount of phosphate ions exist in the electrolytic solution, and have disclosed, in WO00/55876, addition to a water-containing electrolytic solution a compound that generates a phosphate ion in an aqueous solution and a chelating agent that forms an aqueous aluminum chelate complex by coordination with aluminum, in order to produce a combined product of an aqueous aluminum chelate complex and a phosphate ion by reaction of the aluminum ion eluted from the electrode foil and the chelating agent and the phosphate ion. Because the combined product of an aqueous aluminum chelate complex and a phosphate ion maintains chemical equilibrium with the phosphate ions in the electrolytic solution in the state that it is dissolved in the electrolytic solution or attaches to the electrode foil, it can prolong the duration time for the adequate amount of the phosphate ions to exist in the electrolytic solution and prevent the deterioration of the anode and the cathode for a long period.